Mariia Krasikova, Felix Kronowetter, Sergey Krasikov
et al.
Resonant states underlie a variety of metastructures that exhibit remarkable capabilities for effective control of acoustic waves at subwavelength scales. The development of metamaterials relies on the rigorous mode engineering providing the implementation of the desired properties. At the same time, the application of metamaterials is still limited as their building blocks are frequently characterized by complicated geometry and can't be tuned easily. In this work, we consider a simple system of coupled Helmholtz resonators and study their properties associated with the tuning of coupling strength and symmetry breaking. We numerically and experimentally demonstrate the excitation of quasi-bound state in the continuum in the resonators placed in a free space and in a rectangular cavity. It is also shown that tuning the intrinsic losses via introducing porous inserts can lead to spectral splitting or merging of quasi-\textit{bound states in the continuum} and occurrence of \textit{exceptional points}. The obtained results will open new opportunities for the development of simple and easy-tunable metastructures based on Helmholtz resonances.
Electric dipoles and magnetic dipoles are the most fundamental particles in electromagnetic theory. Huygens and Janus sources, formed by the orthogonal combination of electric and magnetic dipoles, both show good directionality in the near field. Although the Huygens source has been widely used in antennas and metasurfaces, the applications of Janus source are heretofore limited. In this paper we report the first physical construction of an active Janus source. Through full-wave simulations within the PPW environment, we show that our source achieves the directional electromagnetic near-field and quasi-isotropic far-field requisite of the Janus source. Using this fact, we demonstrate that two active Janus sources in close proximity (about 0.10 to 0.25 wavelengths) achieve a near 1000-fold reduced mutual coupling compared to electric dipole sources. The achievement of strong mutual coupling suppression and quasi-isotropic radiation make the Janus source an ideal candidate for consideration in future compact MIMO communication systems.
Sebastiano Cominelli, Benjamin Vial, Sébastien Guenneau
et al.
Open cavities are often an essential component in the design of ultra-thin subwavelength metasurfaces and a typical requirement is that cavities have precise, often low frequency, resonances whilst simultaneously being physically compact. To aid this design challenge we develop a methodology to allow isospectral twinning of reference cavities with either smaller or larger ones, enforcing their spectra to coincide so that open resonators are identical in terms of their complex eigenfrequencies. For open systems the spectrum is not purely discrete and real, and we pay special attention to the accurate twinning of leaky modes associated with complex valued eigenfrequencies with an imaginary part orders of magnitude lower than the real part. We further consider twinning of 2D gratings, and model these with Floquet-Bloch conditions along one direction and perfectly matched layers in the other one; complex eigenfrequencies of special interest are located in the vicinity of the positive real line and further depend upon the Bloch wavenumber. The isospectral behaviour is illustrated, and quantified, throughout by numerical simulation using finite element analysis.
Mohamed Farhat, Younes Achaoui, Julio A. Iglesias Martinez
et al.
The confinement of waves in open systems represents a fundamental phenomenon extensively explored across various branches of wave physics. Recently, significant attention has been directed towards bound states in the continuum (BIC), a class of modes that are trapped but do not decay in an otherwise unbounded continuum. Here, we theoretically investigate and experimentally demonstrate the existence of quasi-BIC (QBIC) for ultrasonic waves by leveraging an elastic Fabry-Pérot metasurface resonator. We unveil several intriguing properties of the ultrasound QBIC that are robust to parameter scanning, and we present experimental evidence of a remarkable Q-factor of 350 at around 1 MHz frequency, far exceeding the state-of-the-art using a fully acoustic underwater system. Our findings contribute novel insights into the understanding of BIC for acoustic waves, offering a new paradigm for the design of efficient, ultra-high Q-factor ultrasound devices.
We propose and verify a method for controlling the frequency dependence of the group delay of electromagnetic waves over a broad frequency range using the Brewster effect in single-layer metamaterials with finite thickness, here referred to as metafilms. When the metafilm's reflectance vanishes regardless of the incident frequency, the group delay can be large near its resonance frequency while maintaining the transmittance close to unity regardless of the incident frequency. Furthermore, when several reflectionless metafilms are stacked together, the total group delay should be given as the sum of the individual group delays. In this study, we realize reflectionless metafilms by arranging the meta-atoms so that the Brewster effect occurs regardless of the incident frequency. We evaluate in numerical simulations and experiments the frequency dependence of the transmittance and of the group delay of a three-layer metamaterial composed of reflectionless metafilms with different resonance frequencies, and find that the total transmittance and group delay of this metamaterial agree respectively with the product of the transmittances and the sum of the group delays of the constituent metafilms.
The calculation of electromagnetic field distributions within structured media is central to the optimization and validation of photonic devices. We introduce WaveY-Net, a hybrid data- and physics-augmented convolutional neural network that can predict electromagnetic field distributions with ultra fast speeds and high accuracy for entire classes of dielectric photonic structures. This accuracy is achieved by training the neural network to learn only the magnetic near-field distributions of a system and to use a discrete formalism of Maxwell's equations in two ways: as physical constraints in the loss function and as a means to calculate the electric fields from the magnetic fields. As a model system, we construct a surrogate simulator for periodic silicon nanostructure arrays and show that the high speed simulator can be directly and effectively used in the local and global freeform optimization of metagratings. We anticipate that physics-augmented networks will serve as a viable Maxwell simulator replacement for many classes of photonic systems, transforming the way they are designed.
Urs A. T. Hofmann, Sergio Pérez-López, Héctor Estrada
et al.
Multiple reflections between transducer and imaged object can naturally occur in ultrasound imaging and other acoustic sensing applications such as sonar. The repeated interaction of the emitted wavefront with the imaged object is traditionally regarded an undesired reverberation artifact, often misinterpreted as fictitious acoustic boundaries. Here we introduce high order reflection pulse-echo (HOPE)-ultrasound, a novel method that leverages high order reflections to improve on several aspects of conventional ultrasound imaging. HOPE is experimentally demonstrated to resolve sub-micrometer features by breaking through the Nyquist resolution limit. The major contrast enhancement of the high reflection orders allowed to reveal defects within materials invisible to conventional scanning acoustic microscopy. The technique is further shown to improve accuracy of frequency-dependent ultrasound attenuation measurements from biological tissues. HOPE ultrasound requires no additional hardware and is easy to implement, underscoring its potential to boost imaging performance in a broad range of biomedical imaging, nondestructive testing, and other acoustic sensing applications.
The photogalvanic effect (PGE) occurring in noncentrosymmetric materials enables the generation of a dc photocurrent at zero bias with a high polarization sensitivity, which makes it very attractive in photodetection. However, the magnitude of the PGE photocurrent is usually small, leading to a low photoresponsivity, and therefore hampers its practical application in photodetection. Here, we propose an approach to largely enhancing the PGE photocurrent by applying an inhomogenous mechanical stretch, based on quantum transport simulations. We model a two-dimensional photodetector consisting of the wide-bandgap MgCl$_2$/ZnBr$_2$ vertical van der Waals heterojunction with the noncentrosymmetric $C_{3v}$ symmetry. Polarization-sensitive PGE photocurrent is generated under the vertical illumination of linearly polarized light. By applying inhomogenous mechanical stretch on the lattice, the photocurrent can be largely increased by up to 3 orders of magnitude due to the significantly increased device asymmetry. Our results propose an effective way to enhance the PGE by inhomogenous mechanical strain, showing the potential of the MgCl$_2$/ZnBr$_2$ vertical heterojunction in the low-power UV photodetection.
The coupling effects affecting the vibrations of two close nanostructures (e.g., two metal nanoplates or nanospheres separated by a thin dielectric layer) may considerably alter their vibrational eigenfrequencies, as demonstrated by several recent experimental studies. In this work, we present theoretical investigations of these coupling processes based on a continuum mechanics approach, considering various systems composed by two identical nanostructures mechanically coupled by a spacer made of a different material and computing their eigenfrequencies as a function of the spacer thickness. We first discuss the vibrations of stacked slabs, a one-dimensional problem which can be treated analytically. The more complex configurations of dimers of rods or spheres coupled by a finite cylindrical spacer are then treated numerically. In all cases, the frequency shifts occurring for thin spacers can be simply interpreted as a modification of the boundary conditions of the problem as compared to the single nanostructure case, while those predicted near specific spacer thicknesses are ascribed to an avoided crossing effect, happening when the individual building blocks of the dimers (nanostructures and spacer) present common eigenfrequencies.
A simple expression for the Gamow factor is obtained using a second order curvature corrected tunneling potential. Our results show that it approximates accurately the `exact-WKB' transmission coefficient obtained by numerically integrating over the tunneling region to obtain the Gamow factor. The average difference in current density using the respective transmission coefficients is about $1.5 \%$, across a range of work-functions $φ\in [3-5.5]$eV, Fermi energy ${\cal{E}}_F$ in [5-10]eV, local electric fields $E_l$ in[3-9]eV and radius of curvature $R \geq 5$nm). An easy-to-use correction factor $λ_P$ is also provided to approximately map the `exact-WKB' current density to the `exact' current density in terms of ${\cal{E}}_F/φ$. The average error on using $λ_P$ is found to be around $3.5\%$. This is a vast improvement over the average error of $15\%$ when $λ_P = 1$. Finally, an analytical expression for the curvature-corrected current density is obtained using the Gamow factor. It is found to compare well with the `exact-WKB' current density even at small values of local electric field and radius of curvature.
AbstractIn this study, we investigated the design and construct of a chitosan (CA)‐based targeted gene delivery system and evaluated its function. To this end, CA–folic acid/pDNA (CA–FA/pDNA) nanoparticles were prepared in different formulations using the ion gelation method. All the synthesized nanoparticles were characterized using FTIR, TEM, SEM and DLS. Moreover, the effects of molecular weight (MW) of CA, DNA, and CA concentration were inspected on encapsulation efficiency (EE). The results showed that the EE of pDNA was directly proportional with MW of CA and CA concentration but was in an inverse proportion with DNA concentration. In addition, high MW of CA and low MW of CA nanoparticles showed lower and higher pDNA release in all pH ranges, respectively. It is concluded that the N/P ratio increase can cause controlled pDNA release.
JASWANTH GOWDA B. H., S. J. SHANKAR, MURALI MUNISAMY
et al.
Objective: To develop two different oral formulations such as 5-fluorouracil (5-FU) tablets and oxaliplatin (OX) microspheres which were further filled into capsules and coated with pH-sensitive polymer (eudragit S-100) for the chronotherapeutic treatment of colon cancer (Fluorouracil: Oxaliplatin regimen) to perform as a substitute for intravenous (IV) route based chronomodulated chemotherapy.
 Methods: The 5-FU tablet formulation was prepared with alginate and guar gum polymers in varied concentrations using wet granulation technique in two varieties such as granules coated and tablet coated formulations using eudragit RSPO as coating material to achieve controlled drug release. Alongside OX microspheres were formulated using the ionotropic gelation methodology in combination with alginate and chitosan polymers in varying concentrations to accomplish a time-controlled drug release. Prepared formulations were evaluated for pre-compression and post-compression parameters, percentage yield, percentage drug entrapment, Fourier transformed infrared spectroscopy (FT-IR), Differential scanning calorimetry (DSC), Scanning electron microscopy (SEM), In vitro and Ex vivo dissolution studies.
 Results: Pre-compression and post-compression parameters for 5-FU tablets were satisfied with Indian pharmacopeia specifications. The entrapment efficiency of OX microspheres were increased due to the elevated concentration of polymers up to a certain level as seen in A7M, further greater the concentration of polymer resulted in a decline of entrapment efficiency as seen in A4M and A8M. The optimized formulations A14T and A14M were shown in vitro drug release of 90.36 % by 24 h and 79.63 % by 9 h respectively.
 Conclusion: The two different oral formulations of 5-FU (Tablets) and OX (Microspheres) were found to be successful in controlled drug release. Therefore they can be efficiently used to control the rate of drug release to the colon in synchronization with the circadian timing system in the belief of improved therapeutic efficacy, tolerability and overall survival rate of cancer patients. Hence it is promised to be a better alternative for intravenous route based chronomodulated chemotherapy.
van Son Nguyen, Laurent Badie, Emmanuel Senechault
et al.
This work presents for the first time a flexible over-moded resonator (OMR) based on P(VDF-TrFE) thin films. The devices were manufactured on commercially available elastic substrate with inkjet-printed electrodes. The sensing copolymer films used in the devices were polarized by the corona method after electrode deposition. The main performance parameters of the component were then determined. The manufactured OMRs on P(VDF-TrFE) exhibited a linear variation of frequency versus temperature and a very large value of temperature coefficient of frequency (TCF > 1600 ppm/degrees C). These properties suggest a great potential for using such components as low-cost and high-precision temperature sensors. The electromechanical coupling coefficient and the quality factor of the resonator were also characterized versus temperature.
Waqas W. Ahmed, Mohamed Farhat, Xiangliang Zhang
et al.
Concealing an object from incoming waves (light and/or sound) remained science fiction for a long time due to the absence of wave-shielding materials in nature. Yet, the invention of artificial materials and new physical principles for optical and sound wave manipulation translated this abstract concept into reality by making an object acoustically invisible. Here, we present the notion of a machine learning-driven acoustic cloak and demonstrate an example of such a cloak with a multilayered core-shell configuration. Importantly, we develop deterministic and probabilistic deep learning models based on autoencoder-like neural network structure to retrieve the structural and material properties of the cloaking shell surrounding the object that suppresses scattering of sound in a broad spectral range, as if it was not there. The probabilistic model enhances the generalization ability of design procedure and uncovers the sensitivity of the cloak parameters on the spectral response for practical implementation. This proposal opens up new avenues to expedite the design of intelligent cloaking devices for tailored spectral response and offers a feasible solution for inverse scattering problems.
Mohammad Jobayer Hossain, Bibek Tiwari, Indranil Bhattacharya
A high theoretical efficiency of 47.2% was achieved by a novel combination of In0.51Ga0.49P, GaAs, In0.24Ga0.76As and In0.19Ga0.81Sb subcell layers in a simulated quadruple junction solar cell under 1 sun concentration. The electronic bandgap of these materials are 1.9 eV, 1.42 eV, 1.08 eV and 0.55 eV respectively. This unique arrangement enables the cell absorb photons from ultraviolet to deep infrared wavelengths of the sunlight. Emitter and base thicknesses of the subcells and doping levels of the materials were optimized to maintain the same current in all the four junctions and to obtain the highest conversion efficiency. The short-circuit current density, open circuit voltage and fill factor of the solar cell are 14.7 mA/cm2, 3.38 V and 0.96 respectively. In our design, we considered 1 sun, AM 1.5 global solar spectrum.
Field emisison of electrons crucially depends on the enhancement of the local electric field around nanotips. The enhancement is maximum when individual emitter-tips are well separated. As the distance between two or more nanotips decreases, the field enhancement at individual tips reduces due to the shielding effect. The anode-proximity effect acts in quite the opposite way, increasing the local field as the anode is brought closer to the emitter. For isolated emitters, this effect is pronounced when the anode is at a distance less than three times the height of the emitter. It is shown here that for a large area field emitter (LAFE), the anode proximity effect increases dramatically and can counterbalance shielding effects to a large extent. Also, it is significant even when the anode is far away. The apex field enhancement factor for a LAFE in the presence of an anode is derived using the line charge model. It is found to explain the observations well and can accurately predict the apex enhancement factors. The results are supported by numerical studies using COMSOL Multiphysics.
Marco Nardone, Yasas Patikirige, Kyoung E. Kweon
et al.
Temporal variations of Cu(In,Ga)Se$_2$ photovoltaic device properties during light exposure at various temperatures and voltage biases for times up to 100 h were analyzed using the kinetic theory of large lattice relaxations. Open-circuit voltage and p-type doping increased with charge injection and decreased with temperature at low injection conditions. Lattice relaxation can account for both trends and activation energies extracted from the data were approximately 0.9 and 1.2 eV for devices with lower and higher sodium content, respectively. In these devices, increased sodium content resulted in higher initial p-type doping with greater stability. First principles calculations providing revised activation energies for the ($V_{Se}-V_{Cu}$) complex suggest that this defect does not account for the metastability observed here.
Rejection of unmitigated vibrational disturbances represents an ongoing dilemma in complex linkage systems. In this work, we present an inherent and self-reliant vibration isolation mechanism in architected periodic chains of serially pivoted pendulums. Absorption of external excitations is achieved by virtue of Bragg-type band gaps which stem from the emergent chain dynamics. Owing to its coupled dynamics, the self-repeating cell of a Phononic Crystal (PC) is not trivial and cannot be readily identified from the periodic arrangement by inspection. As such, this work entails the extraction of a "pseudo" unit cell of an equivalent PC lattice from the derived motion equations of a finite chain, which departs from traditional wave dispersion methods. The model presented herein comprises a chain of linked pendulums with periodic variations of inertial/geometrical properties, carrying a payload at one end. We ultimately show evidence of forbidden wave propagation in prescribed frequency regimes, reminiscent of band gaps in PC lattices. The presented framework can be invaluable in applications that require vibration reduction in the delivery of payloads including gantry cranes, robotic arms and space tethers.
Andrey Sosorev, Dmitry Maslennikov, Elizaveta Feldman
et al.
Rapid progress in organic electronics demands new highly efficient organic semiconducting materials. Nevertheless, only few materials have been created so far that show reliable band-like transport with high charge mobilities, which reflects the two main obstacles in the field: the poor understanding of charge transport in organic semiconductors (OSs) and the difficulty of its quantification in devices. Here, we present a spectroscopic method for assessment of the charge transport in organic semiconductors. We show that the intensities of the low-frequency Raman spectrum allow calculation of the dynamic disorder that limits the charge carrier mobility. The spectroscopically evaluated mobility clearly correlates with the device charge mobility reported for various OSs. The proposed spectroscopic method can serve as a powerful tool for a focused search of new materials and highlights the disorder bottleneck in the intrinsic charge transport in high-mobility organic semiconductors.
Shaokang Wang, Thomas F. Carruthers, Curtis R. Menyuk
Lowering the noise level of short pulse lasers has been a long-standing effort for decades. Modeling the noise performance plays a crucial role in isolating the noise sources and reducing them. Modeling to date has either used analytical or semi-analytical implementation of dynamical methods or Monte Carlo simulations. The former approach is too simplified to accurately assess the noise performance in real laser systems, while the latter approach is too computationally slow to optimize the performance as parameters vary over a wide range. Here, we describe a computational implementation of dynamical methods that allows us to determine the noise performance of a passively modelocked laser within minutes on a desktop computer and is faster than Monte Carlo methods by a factor on the order of 1000. We apply this method to characterize a laser that is locked using a fast saturable absorber---for example, a fiber-based nonlinear polarization rotation---and a laser that is locked using a slow saturable absorber---for example, a semiconductor saturable absorbing mirror.